Photolithography tools are indispensable in the manufacturing of microelectronic devices. As a photolithography tool typically has a very limited field of view, before a wafer is exposed on a stage of the photolithography tool, it must be pre-aligned in order to be located within the field of view of the photolithography tool. Therefore, pre-alignment of the wafer constitutes an important part of the exposure process performed by the photolithography tool on the wafer.
Generally, pre-alignment can be done in a mechanical or optical manner. Optical pre-alignment can provides a higher precision over the mechanical approach, but it is expensive. An optical pre-alignment process involves detecting an edge and notch in the wafer using precision optics, centering the wafer according to an algorithm and detecting the wafer using linear charge-coupled devices (CCDs) as sensors.
Commonly-used devices for optical pre-alignment can be divided into two types.
The first type is worked in a transmissive manner, as shown in FIG. 1, a lighting unit and an imaging unit are arranged on both sides of an object under measurement, i.e., the wafer. As the object under measurement does not allow transmission of light, its boundary can be imaged on the CCD detector, and the position (e.g., including vertical and horizontal) and attitude (e.g., a tilt) of the wafer can be calculated based on an image of the boundary. In practical use, this approach is mainly associated with two issues: 1) when the object under measurement is a bonded wafer with its notch or flat remaining untreated, the light will be blocked and the wafer's attitude cannot be ascertained; and 2) the image quality is poor for glass substrates.
Another type is worked in a reflective manner, as shown in FIG. 2, the wafer notch can be identified by taking advantage of different reflectivities of the substrate (i.e., a carrier wafer) and wafer. However, the reflective pre-alignment device is associated with two major issues: 1) there are usually small profile irregularities at the wafer notch resulting from the bonding or other processes, which tend to impair the imaging quality and reliability of the device; and 2) the reflective pre-alignment device is susceptible to interference. After the wafer is placed on a chuck 1, as shown in FIG. 3, annular grooves 1a, 1b and 1c in the chuck 1 will also be imaged on the CCD detector, making it difficult to identify the wafer edge from the captured image (as shown in FIG. 4).
Patent Publication No. CN102402127A, entitled “Wafer Pre-aligner and Method Therefor”, provides a measurement device utilizing a height difference. However, this device does not take into account the relationship between vertical and horizontal magnifications and is hence insufficient in resolution. In addition, the calibration disclosed in the patent fails to provide horizontal measurements.